1. Field of the invention
The present invention relates to a spark plug for use in internal combustion engines.
2. Description of the Related Art
Conventional spark plugs for use in internal combustion engines such as automotive engines have the igniting portion formed of a platinum (Pt) alloy chip welded to the tip end of an electrode in order to improve its resistance to spark consumption. However, in view of the high cost of platinum, it has been proposed to use less expensive iridium (Ir) as a chip material.
A problem with the use of Ir as a material to constitute the igniting portion of the spark plug is that Ir is easy to oxidize and evaporate in a high temperature range of 900° to 1,000° C. Therefore, if it is directly used in the igniting portion of the electrode, it is more consumed by oxidation and evaporation than by spark. Therefore, the spark plug using Ir in the igniting portion of an electrode is highly durable using low-temperature conditions as in driving on city roads but their endurance drops considerably during continuous running at high speed.